Finicky snails provide new
clues to the evolution of coastal ecosystems

BY MARK SHWARTZ

Whether sautéed in wine or steamed in the shell,
mussels have long been a favorite of seafood lovers. For most
people, the type of mussel served isn't important -- as long as
there are plenty of them on the plate.

But
in the wild, it's a different matter. According to a new study in
the journal Science, when it comes to preying on mussels,
marine snails are often pickier than people.

The
study, led by scientists from Stanford University, focused on a
species of mussel that California snails love to eat but Oregon
snails won't come near. The scientists discovered that this
culinary preference is probably an inherited trait -- the result of
generations of genetic and geographic isolation along the shores of
the Pacific.

The
discovery of finicky snail populations on the Oregon and Washington
coast could have profound implications for managing marine
ecosystems worldwide, the researchers added.

"If
you go down the coast from Canada to Mexico, you will find species
that individually look the same but actually have undergone genetic
adaptations to local conditions," said George N. Somero, the David
and Lucile Packard Professor in Marine Science at Stanford and
co-author of the Science study. "As a result, a species
that's relatively unimportant in one habitat may turn out to be
very important in another."

The
new findings, published in the May 16 issue of Science, were
based on experiments conducted at Stanford's Hopkins Marine
Station, where researchers analyzed the eating habits of the
channeled whelk (Nucella canaliculata) -- an inch-long snail
commonly found in coastal waters from Alaska to California. Whelks
are voracious consumers of mussels, despite being much smaller than
their prey.

"A
whelk drills through the shell of a mussel using its file-like
tongue -- the radula -- and acid secretions," said Eric Sanford,
lead author of the Science study. "When the hole is drilled
through, a tube called the proboscis is extended into the shell,
and the soft mussel tissue is ingested."

Scientist Eric
Sanford surveyed the impact of snail predation on mussels in the
intertidal zone at Soberanes Point in central California.
Photo: Jason
Podrabsky

Sanford, now a research associate at Brown University, led the
study while he was a postdoctoral fellow in Somero's laboratory at
Hopkins.

In
one experiment, researchers fed whelks a diet consisting
exclusively of sea mussels (Mytilis californianus) -- a
species that is abundant along the Pacific coast. The reaction was
decidedly mixed: The vast majority of whelks from Oregon and
Washington refused to eat the sea mussels, while California whelks
eagerly preyed on them.

These laboratory results mirrored what the research team had
already discovered in the wild.

"We
found that, along the California coast, channeled whelks feed
intensely on sea mussels," Sanford explained. "But in Oregon, sea
mussels are a meal that few whelks will touch, preferring instead
to prey on bay mussels (Mytilis trossulus), which are less
common in California. Remarkably, in our experiment, we have found
that these differences in predatory behavior appear to have a
genetic basis."

Genetic isolation

Unlike many marine invertebrates, which release offspring into
the open sea, channeled whelks attach their egg capsules to nearby
rocks. This breeding behavior led Sanford to speculate that newborn
whelks probably hatch and grow up within a few yards of where their
parents lived. By staying close to home, whelk populations
eventually would become isolated from one another, he proposed,
resulting in dozens of genetically distinct communities of whelks
up and down the West Coast.

To
test this hypothesis, Sanford collected whelks from 13 sites along
a 900-mile stretch of the Pacific -- from Southern California to
the northern tip of Washington State. He then brought the whelks
back to the lab, where DNA analysis confirmed that very little
interbreeding had taken place among the 13 populations
sampled.

"When we did the genetics, we found that the snails are a
single species that is reproductively isolated into separate
populations," he said. "Generation after generation has lived and
died, isolated on their own particular stretch of rocky
coastline."

Culinary divide

If
all channeled whelks belong to the same species, then why is there
such a sharp difference in the feeding behaviors of northern and
southern populations? Do whelks born and raised in the Pacific
Northwest inherit their dislike for sea mussels, or is it a learned
behavior?

To
find out, the scientists decided to raise baby whelks in the lab
using egg capsules Sanford had collected from eight sites in
California, Oregon and Washington.

"Once the juvenile snails hatched from their capsules, we
raised them on a common diet of bay mussels until they reached
adult size about 10 months later," he recalled. "We then tested
whether these lab-reared whelks would drill sea mussels -- a
species that they had never encountered before."

The
results were dramatic and virtually identical to what had been
observed in the field: 75 percent of snails raised from California
eggs drilled into the shells of sea mussels, compared to only 7
percent of the Oregon and Washington hatchlings.

"Since the snails were raised under identical laboratory
conditions, this strongly suggests that the differences in drilling
behavior have a genetic basis," Sanford concluded, noting that
these behavioral differences may have evolved in response to
regional differences in prey availability. Bay mussels are scarce
in California, but sea mussels are plentiful. Therefore, natural
selection would favor California snails that feed on sea
mussels.

Conservation implications

The
discovery of distinct whelk populations along the Pacific coast has
significant implications for the design and management of marine
reserves, Sanford noted.

"Is
a snail just a snail wherever it is found, or are there important
evolutionary differences that determine what role it will play in a
coastal ecosystem?" he asked. "If you studied whelks in Oregon, you
would conclude that, although they certainly affect some species,
such as bay mussels, they probably don't play an essential role in
the overall community."

In
contrast, he noted, the California whelk is a predator on sea
mussels -- a vital member of the rocky shore community: "If not
held in check, sea mussels often are the dominant competitor --
overgrowing and outcompeting barnacles, algae and other species for
limited space on the shore."

Earlier studies have shown that sea mussel populations are
kept under control by another rapacious predator -- the ochre sea
star (Pisaster ochraceus).

"The sea star is a keystone species," Sanford explained. "As
long as it's there, you'll have diversity. But if you take the sea
star out, then the sea mussels can move downshore and crowd out
other species living on the rock."

If
sea star populations were suddenly reduced by disease or some other
disturbance, would the channeled whelk fill the void and help keep
sea mussels in check?

To
find out, the research team placed sea mussels in mesh cages
specially designed to exclude sea stars, then attached the cages to
intertidal sites in California and Oregon. After nine months,
whelks had discovered and drilled nearly 20 percent of the caged
mussels in California.

"In
contrast," the authors wrote, "not a single mussel was eaten in
Oregon, despite mean whelk densities that were seven times greater
than in California." Given the slow growth and low supply of young
sea mussels in California, the authors concluded that whelks could
partially or completely fill the sea star's predatory role in
California -- but not in Oregon.

"A
species that seems only marginally important in one community may
turn out to be an essential player in a community just a few
hundred miles down the coast," Sanford concluded. "Thus, scientists
and resource managers have to be careful in assuming that local
studies of species interactions apply everywhere that the species
co-occur. These results are a strong argument for conserving and
maintaining whole, intact marine communities, rather than focusing
on certain target species."

Other coauthors of the Science study are Melissa S.
Roth, a former Stanford undergraduate now with the U.S. Fish and
Wildlife Service; Glenn C. Johns, a Stanford postdoctoral fellow;
and John P. Wares, a postdoctoral fellow at the University of
California-Davis.

This study was part of the Partnership for Interdisciplinary
Studies of Coastal Oceans (PISCO) -- a consortium of marine
scientists from Stanford, Oregon State University, the University
of California-Santa Barbara and the University of California-Santa
Cruz -- funded by the David and Lucile Packard Foundation.
Additional support was provided by the Andrew W. Mellon Foundation,
the National Science Foundation and the Undergraduate Research
Opportunities program at Stanford.